The PNGs you want are in: /Applications/DEVONthink Pro.app/Contents/PlugIns/SorterPlugin.bundle/Contents/MacOS/DEVONthink Sorter.app/Contents/Resources
I found the default text distracting, so mine are now a simple gray tab.
(Unfortunately, this doesn’t work for Together.app, which only lets you edit the bounding box edges. The text appears to be drawn by the app [or, at least, doesn't seem to be a bitmap in Resources.])